To obtain joining strength with high strength and less dispersion by adding a specified ratio of B into sprayed coating composed of a Ni based alloy containing a specified ratio of Cr and Si.

The thermal spraying material forming sprayed coating film is a Ni based alloy containing, by mass, 18.0-19.5% Cr, 7.0-10.5% Si, and 2-7% B. For brazing the alloy of this component, heating up to a temperature ≥1130°C is preferable, but B reacts on the oxide of Cr and Si and the melting point of the resultant oxide is low, so that heating ≥1130°C causes the resultant oxide to become liquid and to flow from the inside of the layer of brazing filler metal to the outside. Accordingly, after the sprayed coating film is melted, the amount of oxide remained inside the layer of brazing filler metal gets very small and the joining strength becomes high. Sprayed coating film 3 is formed on both sides of separate plates 2, a fin stock 4 is corrugated into a fin 1, and then both of them are assembled and subjected to thermal joining. Or, the sprayed coating film 3 is formed into the fin 1 and then subjected to thermal joining together with the separate plates 2, as well.